Assembly of large base structures such as space stations, drilling platforms, and precision clearance machinery requires a strict accounting of assembly hardware to ensure that stray hardware does not become lost, possibly causing damage to the base structure. For example, a nut or bolt drifting away from a space station may become a long-term projectile hazard to subsequent spacecraft in the vicinity of the space station. A bolt dropped into a reduction gear may cause damage to the gears which leads to jamming of the machinery, rendering it inoperable. In an elevated application such as skyscraper construction, a dropped fastener could be dangerous to people or equipment underneath, as well as inconvenient for a user who must carry extra fasteners in case of loss. A fastener that falls into some portion of a spacecraft could cause an on-orbit failure of an antenna or solar array deployment, potentially making a multi-million dollar spacecraft useless. A fastener falling into a jet engine, aircraft landing gear retraction system, or flight control mechanism could cause a life threatening failure. A fastener lost from a ribcage separation fixture could be left behind inside an open heart surgery patient. In addition to “permanently installed” fasteners, there is also a need for accountability or control of fasteners for equipment that is used temporarily and/or repetitively with or near sensitive equipment. Retrieving fasteners or similar hardware after they have been dropped or otherwise lost is often difficult, if not impossible.
The problem of lost or misplaced fasteners can cause significant inconvenience and expense for base structures in underwater, outer space, and other isolated or unusual locations. Accordingly, tethered fasteners have become common for use in any environments in which it is desirable to prevent a fastener from becoming freely detached from a base structure.
Most tether methods involve some modification to the fastener, such as by the creation of a groove at the base of the threads or in the head. This modification can cause delays and expense in manufacturing, can compromise the corrosion protection coating/treatment, and can weaken the fastener. Additionally, this modification must be documented on the engineering drawings for each fastener modified—costing additional time and effort—and requires that these specialty fasteners, which might not be readily available in the field, to be used for proper tethering.
Existing tether methods that do not require fastener modification involve placing the fastener into a hole toward one end of a large rectangular sheet metal “washer” and then bending the other end of this makeshift “washer” over the top of the fastener head to prevent its escape. The result is a large “C-shaped” bracket hanging over the fastener head. By design, this bracket must be easy enough to bend so that the fastener can be inserted and the bracket bent over the fastener head (usually by hand), but this malleability makes the bracket weak enough to be accidently bent or even broken by casual contact. This “C-shaped” bracket also has an inconvenient tendency to snag on clothing, cables, lanyards, tethers, and other nearby structures which brush against the bracket during movement, thus causing damage to the bracket and/or the contacting structure and possibly negating any “tethering” function of the bracket. Finally, the “C-shaped” bent-washer bracket is difficult to unbend and re-use if the fastener must be removed and replaced, so is a one-time-use item which itself may result in unwanted extra debris or free-floating objects (e.g., in undersea or outer space use environments).